Further study of the evolution and decline of the normal progestational endometrium of the rabbit will be undertaken. The six phases already defined, viz. phase 1 (priming), phage 2 (proliferative, phase 3 (folding and secretion), phase 4 (cell fusion), phase 5 (maximally progestational), and phase 6 (declining) will be analyzed by electron microscopic and further light microscopic means. Particular analyzed by electron microscopic and further light microscopic means. Particular emphasis will be placed on the manner and tempo of cell fusion, differential responses of the antimesometrial and mesometrial surfaces of the uterine lining, the significance of secretory activity, the origin and sighificance of ciliated cells, and the mechanism by which the progestational epithelium is restored to the resting state. A hormonal analysis of the phases of endometrial development and involution will be carried out, with special reference to the roles of estradiol, progesterone and 20 alpha. The proposed work is expected to lay a foundation for the later study of the cellular interactions between the maternal and embryonic tissues at the time of implantation (about the 7th day). Such a definition of cellular responsiveness will provide a rational basis for the development of new contraceptive agents and for the possible development of a more sensitive and perhaps quantitative assay for the biologically active sex steroids.